The present invention is directed to compositions particularly suitable for electrophotographic and electrographic liquid development and for ink jet printing. More specifically, the present invention is directed to a process for preparing colored silica particles and to ink jet ink compositions and liquid developer compositions containing these particles. One embodiment of the present invention is directed to an ink jet ink composition comprising a liquid vehicle, a thickening agent, an optional conductivity enhancing agent, and a plurality of colored particles comprising hydrophilic silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. Another embodiment of the present invention is directed to a liquid developer composition which comprises a liquid medium, a polymeric material soluble in the liquid medium, a charge control agent, and a plurality of colored particles comprising hydrophilic silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. Still another embodiment of the present invention is directed to a liquid developer composition which comprises a liquid medium, a resin, a charge control agent, and a plurality of colored particles comprising hydrophilic silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. Yet another embodiment of the present invention is directed to an ink jet ink composition comprising water, a solvent, and hydrophilic silica particles having dyes covalently bonded to the particle surfaces through silane coupling agents. The colored silica particles in each composition can be prepared by reacting hydrophilic silica particles with a silane coupling agent in the absence of water to form silica particles with the coupling agents covalently bonded thereto, followed by reacting a dye with the coupling agent. Another embodiment of the present invention is directed to a process for generating images by ink jet processes with the ink compositions illustrated herein. Still another embodiment of the present invention is directed to a process for forming images and developing the images with the liquid developer compositions illustrated herein.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, entails placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image may subsequently be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Development of electrostatic latent images with liquid developer compositions is also known. Liquid electrophotographic developers generally comprise a liquid vehicle in which is dispersed charged colored toner particles. In liquid development processes, the photoreceptor bearing the electrostatic latent image is transported through a bath of the liquid developer. Contact with the charged areas of the photoreceptor causes the charged toner particles present in the liquid vehicle to migrate through the liquid to the charged areas of the photoreceptor to develop the latent image. Thereafter, the photoreceptor is withdrawn from the liquid developer bath with the charged pigment particles adhering to the electrostatic latent image in image configuration. The developed image is then transferred to a suitable substrate, such as paper or transparency material, and, optionally, may be fixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
In addition, liquid development of electrostatic latent images on charged papers is known. In these processes, the electrostatic latent image, which is usually formulated on a single sheet of dielectric paper, is transported through a bath of the liquid developer. Contact with the liquid developer causes the charged toner particles present in the liquid developer to migrate through the liquid vehicle to the dielectric paper in the configuration of the latent image. Thereafter, the sheet is withdrawn from the liquid developer bath with the charged toner particles adhering to the electrostatic latent image in image configuration. The thin film of residual developer remaining on the surface of the sheet is then evaporated within a relatively short time period, usually less than 5 seconds. Subsequently, the marking pigment particles may optionally be fixed to the sheet by any suitable method.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described, for example, in U.S. Pat. Nos. 4,601,777; 4,251,824; 4,410,899; 4,412,224; and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Another process for thermal ink jet printing entails selectively heating an ink in a reservoir with a series of heaters and with a slit situated on the surface facing the recording substrate. Activating the heaters in imagewise fashion results in the ink being selectively ejected from the slit in those areas where heat was applied, generally as a result of lowered viscosity of the ink in the area to which heat was applied. An electrode assist can be situated behind the printing substrate to attract a conductive ink to the substrate; ink is attracted from the slit to the substrate in areas to which heat has been applied, and the ink remains in the reservoir in areas to which heat has not been applied. This printing process is described in, for example, U.S. Pat. Nos. 4,751,531; 4,710,780; 4,751,533; 4,748,45; 4,737,803; and 4,580,148, the disclosures of each of which are totally incorporated herein by reference.
Ink jet inks are also known. For example, U.S. Pat. No. 4,197,135 discloses an ink with a pH of 8 or more for use in ink jet printers, which contains a water soluble dye and a polyamine containing 7 or more nitrogen atoms per molecule. In addition, U.S. Pat. No. 4,210,916 discloses an ink composition for use in jet printing comprising an aqueous solution of a water-soluble dye and a humectant consisting of at least one water-soluble alkene diol or alkene polyol compound. Further, U.S. Pat. No. 4,685,968 discloses a process for preparing an aqueous-based ink composition for use in ink jet printers, which comprises forming a solution of a dye having at least one negatively charged functional group per molecule, acidifying the solution, cycling the solution through a reverse osmosis membrane to form a concentrate and a permeate, the concentrate including a cation of the compound associated with at least one functional group on the dye and the permeate including a cation formerly associated with at least one functional group, adding water as necessary, concentrating the dye by reverse osmosis, and admixing the concentrated dye with at least one glycol ether. Another patent, U.S. 4,689,078, discloses a recording liquid suitable for ink jet recording comprising a liquid composition containing a disperse dye in which the purity of the disperse dye is 90 percent or higher.
Heterophase ink jet inks are also known. For example, U.S. Pat. No. 4,705,567 discloses a heterophase ink jet ink composition which comprises water and a dye covalently attached to a component selected from the group consisting of poly(ethylene glycols) and poly(ethylene imines), which component is complexed with a heteropolyanion. In addition, U.S. Pat. No. 4,597,794 discloses an ink jet recording process which comprises forming droplets of an ink and recording on an image receiving material by using the droplets, wherein the ink is prepared by dispersing fine particles of a pigment into an aqueous dispersion medium containing a polymer having both a hydrophilic and a hydrophobic construction portion. The hydrophilic portion constitutes a polymer of monomers having mainly polymerizable vinyl groups into which hydrophilic portions such as carboxylic acid groups, sulfonic acid groups, sulfate groups, and the like are introduced. Pigment particle size may be from several microns to several hundred microns. The ink compositions disclosed may also include additives such as surfactants, salts, resins, and dyes.
In addition, processes for the production of colored silica particles are known. For example, U.S. Pat. No. 4,566,908 discloses an azoic pigment suitable for use in an electrophotographic toner having a silica core comprising a core of a fine powder of silica having a particle diameter of not more than 10 microns and a coating of a mono- or polyazoic dye chemically bound to the surface of the silica core through an aminosilane coupling agent. The process for preparing these colored silica particles is detailed at columns 8 to 18 of the patent. In addition, R. Ledger and E. Stellwagen, "Preparation and Analysis of Reactive Blue 2 Bonded to Silica Via Variable Spacer Groups," Journal of Chromatography, vol. 299, pages 175 to 183 (1984), discloses processes for preparing colored silica particles by covalently attaching Reactive Blue 2 dye to silica particles through various spacer groups. The disclosure of this article is totally incorporated herein by reference.
Further, colored polymeric particles having a dye covalently attached to the polymeric particles are illustrated in, for example, copending application U.S. Ser. No. 143,790, entitled "Process for Preparing Colored Particles and Liquid Developer Compositions Thereof". Processes for preparing silica based charge enhancing additives wherein a tetraalkoxysilane is reacted with an alcoholic alkaline solution in the presence of a soluble silica based charge enhancing additive are also disclosed in copending application U.S. Ser. No. 07/214,351, filed Jul. 1, 1988, entitled "Processes for the Preparation of Silica Containing Charge Enhancing Additives" with the named inventors Francoise M. Winnik and Yves Deslandes. Additionally, West German Patent Publication DE-3,330,380 discloses alkoxyaminosilanes which are chemically reacted with free silanol groups.
Of background interest are U.S. Pat. Nos. 2,876,119; 2,993,809; 3,939,087; 4,179,537; and 4,204,871.
Although the above described developers, ink jet inks, and processes for preparing colored silica are suitable for their intended purposes, a need continues to exist for heterophase ink jet inks that exhibit improved waterfastness, reduced feathering, and compatibility with plain paper. A need also exists for heterophase ink jet inks wherein the particles are thermally stable. Further, there is a need for ink jet inks available in a wide variety of colors. There is also a need for ink jet inks with reduced toxicity. In addition, a need continues to exist for simple and economical processes for preparing colored particles suitable for heterophase ink jet inks. A need also exists for liquid developer compositions available in a wide variety of colors. Further, there is a need for liquid developers wherein developers of different colors can be prepared with the same charge control agent. In addition, there is a need for liquid developers wherein the particle size and particle size distribution can be well controlled. There is also a need for liquid developers that afford excellent fixing characteristics to paper. Further, there is a need for recording liquids containing colorant particles with a high degree of transparency, thereby enhancing color quality and enabling the formation of high quality full color images by sequentially applying images of primary colors to a single substrate, each successive image being applied on top of the previous image.